1. Field of the Invention
This invention relates to a differential signal cable and a production method therefor.
2. Description of the Related Art
In as high speed signal transmission as a few Gbps or higher, differential signaling using a differential signal cable has been used. In the differential signaling, signal transmission and reception is performed by transmitting 180 degrees out of phase differential signals to two paired inner conductors respectively at a transmitting end, and taking a difference between the two signals received at a receiving end.
The differential signal cable at least includes the two inner conductors, an insulator, which covers the two inner conductors separately or together, and an outer conductor, which is provided in such a manner as to cover a circumference of the insulator.
Now, currents flowing in the two inner conductors of the differential signal cable can be decomposed into a differential mode, in which the signals are 180 degrees out of phase, and a common mode, in which the signals are in phase.
Because in the ideal differential signaling, the differential mode is input at the transmitting end, and is detected at the receiving end, the differential signal cable is required to minimize a quantity of energy conversion, in other words, mode conversion from the differential mode to the common mode in signal propagation from the transmitting end to the receiving end.
However, in the practical differential signal cable, it is known that the unintended mode conversion occurs due to a difference in length between the two inner conductors, a difference between signal propagation velocities in the two inner conductors, etc.
Such a mode conversion is considered to be caused by a difference between times taken by the signals to propagate in the two inner conductors, in other words, a skew. For that reason, for the differential signal cable for as relatively low speed transmission as lower than a few Gbps, the skew in step response waveform has been measured as a quantitative measure of the mode conversion by using a time domain reflectometer (TDR).
The skew of the differential signal cable is represented by the following formula.
                              Skew          ⁢                                          [          ps          ]                =                ⁢                              t            ⁢                                                  ⁢                          (              P              )                                -                      t            ⁢                                                  ⁢                          (              N              )                                                              =                ⁢                              Δ            ⁢                                                  ⁢                          S              /              c                        ×                                          ɛ                eff                                  1                  /                  2                                            _                                +                                                    S                _                            /              c                        ×                          Δ              ⁡                              (                                  ɛ                  eff                                      1                    /                    2                                                  )                                                        
Here, t(P), t(N): the propagation times in the inner conductors respectively
ΔS: the difference in length between the inner conductors
c: the speed of light in vacuum
S: the average value of the lengths of the inner conductorsεeff1/2=(εeff1/2(P)+εeff1/2(N))/2Δ(εeff1/2)=εeff1/2(P)−εeff1/2(N)
εeff1/2(P), εeff1/2(N): the respective single-ended effectiveness dielectric constants of the inner conductors.
Thus, reducing the difference ΔS in length between the inner conductors and the difference Δ(εeff1/2) in square root of the effectiveness dielectric constant between the inner conductors allows for reducing the skew and suppressing the mode conversion.
On the other hand, for the differential signal cable for as high speed transmission as a few Gbps or higher, the skew cannot precisely be evaluated with the TDR, and therefore an SCD21 (dB), which is one component of a mixed S parameter, has been used as the quantitative measure of the mode conversion.
The SCD21 is for directly expressing the quantity of energy conversion from the differential mode to the common mode in the signal propagation from the transmitting end to the receiving end, and is typically measured in a frequency region to be used using a network analyzer for high frequency measurement. The SCD21 can be made small by making ΔS and Δ(εeff1/2) small.
Note that as prior art publication information associated with the invention of this application, there are the following.
Refer to JP-A-2013-157309 and C. Paul, “Introduction to Electromagnetic Compatibility,” WILEY-INTERSCIENCE, A JOHN WILEY & SONS, INC. PUBLICATION, December 2005, for example.